Catalyst Recovery
Homogeneous catalysts are generally more selective than heterogeneous catalysts but they are harder to separate from the product. The Jessop group, in collaboration with the Eckert and Liotta groups at Georgia Tech, has pioneered several new methods for cleanly recovering and recycling homogeneous catalysts.
1. Perform the catalysis in an ionic liquid and extract the product with supercritical CO2.
The catalyst is not extracted; it remains behind in the ionic liquid and can be used again. (The photo at right shows an orange dye being extracted into supercritical CO2 from an ionic liquid).
2. Perform the catalysis in a liquid polymer and extract the product with supercritical CO2.
See the webpage on liquid polymers.
3. Fluorous Switching.
Adsorb a fluorous catalyst into the coating of fluorous silica particles (left part of the scheme below). The catalyst will not be extracted by many solvents (in yellow) because many solvents are fluorophobic. However, dissolving CO2 into the solvent causes the solvent to expand and become fluorophilic. The catalyst is then extracted into the solvent and can perform homogeneous catalysis. When the reaction is done, one can release the CO2. At that point the catalyst readsorbs onto the fluorous silica and is as easily separated as any heterogeneous catalyst.
